Posted
by
Soulskill
on Saturday October 04, 2008 @01:09AM
from the very-large-yet-very-small dept.

Lumenary7204 writes "The Register has a story about the Large Synoptic Survey Telescope, a project to build a 6.7 meter effective-diameter ground-based telescope that will be used to map some of the faintest objects in the night sky. Jeff Kantor, the LSST Project Data Manager, indicates that the telescope should be in operation by 2016, will generate around 30 terabytes of data per night, and will 'open a movie-like window on objects that change or move on rapid timescales: exploding supernovae, potentially hazardous near-Earth asteroids, and distant Kuiper Belt Objects.' The end result will be a 150 petabyte database containing one of the most detailed surveys of the universe ever undertaken by a ground-based telescope. The telescope's 8.4 meter mirror blank was recently unveiled at the University of Arizona's Mirror Lab in Tucson."

we might be reaching the physical (or practical) limit of data density for hard disk platters, but we'll probably just move to new technologies. it's very unlikely that magnetic disk drives are the pinnacle of data storage technology. there are probably more efficient storage mediums in the works already.

i don't know what can currently match magnetic disks drives in terms of IO speed, but holographic storage [wikipedia.org] shows a lot of promise. in theory, holographic storage can read/write millions of bits of data in pa

Would this mean the end of "bad sectors" as we know it?It would seem to me that if a part of the holographic storage device degrades in some way, one could simply read the data from any number of different "windows" (as described in the Wikipedia article) and get the proper result.

This can be done without holography. The field of science that studies this is information theory [wikipedia.org]. By a proper encoding, one can put redundancy in any data set such that the original information can be recovered, no matter how muc

: "By a proper encoding, one can put redundancy
: in any data set such that the original information
: can be recovered, no matter how much degradation
: there is."

Granted, but according to the related article on channel capacity [wikipedia.org], the Shannon-Hartley theorem indicates that the maximum rate of data transmission across a noisy channel degrades logarithmically as noise increases linearly (if I am reading the article correctly).

Not to mention there is an even easier way,which is simply adding more slots and/or shrinking the sizes of drives. Hell my going on 5 year old machine has slots for 4 drives(2 EIDE and 2 SATA) and we are now seeing plenty of boards with 6 SATA slots or more. And as we have seen drives like chips can still be shrunk a ways. So I kinda doubt we'll be hitting the wall for awhile yet.

That said,where we ARE hitting a wall,and pretty hard at that,is easy to use consumer backup storage. The days of easily backin

The basic problem is that a 6.4 meter aperture can't fit in a launch vehicle, Ares V is only to be 5.5 meters.

Hubble was built at the diameter it was (2.4 meter) because thats about the maximum you could build of a stiff mirror that held its shape well enough through launch to remain optically sound on orbit. When you require 10-nm level precision, it takes a hefty structure to keep things that stiff.

In order to go bigger, the methods they're using for James Webb manage to double the aperture while halving the weight. The way they do this is using active controls and sensors to correct errors rather than rely on avoiding all errors. But looking at James Webb, you'll notice it focuses on IR which is very hard to observe from Earth, while no optical band concept is out there. This is because the new, big Earth-bound scopes use adaptive optics to eliminate seeing errors (the variations in the atmosphere that Hubble avoids), and get potentially better images than Hubble since larger mirrors can be used.

Of course, if the money shows up, there are other advantages to having a space-based observatory, particularly access time and not having to worry about the effectiveness of the adaptive elements, so I'm sure we'll see a proper Hubble replacement eventually, but it's certainly not as critical for scientific progress as some might think.

So consider it a beta... I imagine that the reason its still on the ground is a lack of funding to get it up there... its far cheaper to set it up on the ground, and extremely cheap to fix and maintain in comparison, plus when its on the ground you (they) have essential complete control over it, when its in space you have to let other people play with your toy, or be sacrificed to debris...

I highly doubt this is the be-all, end-all of this telescope (or at least the work the people involved are/will do) or

It would be far less cost effective to upgrade Hubble than to build the LSST. Shuttle launches cost a TON of money not to mention you are risking astronauts lives to try and upgrade something we CAN build better on Earth. Hubble has had optical problems in the past and the 2 flights to repair it cost about one billion dollars. Imagine what a complete rebuild would require.
"Making the most advanced telescope, ON THE GROUND, seems like an oxymoron to me."
And the reason it is considered such an advanced tel

It not being in space might have something to do with the amount of data it would have to transmit and the speed limitations... Besides, you can't replace Hubble, its impossible to exactly replicate that many technical difficulties...

Yes, yes.. the HST has had issues, but all in all, it's been in almost continuous operation for over 18 years. I don't think I've had anything for that long that didn't have problems (including the first wife).

When I worked at the CFHT a few decades ago, they had a bunch of "data reduction" algorithms they ran on each night's run that reduced the amount of data they needed to store by at least a factor of 10.

Honestly, I don't know what the data reduction algorithms did. If I had to guess, I would guess that they included things like a high-pass filter to remove readings that were "in the noise" and maybe some sort of compression for repeated values over large areas like "empty space." Just guessing though.

Back when I was using CFHT there was no high-pass filtering done on the data. That would change the noise properties of the data, which could render the data useless for certain types of analysis. The big space savings were done using lossless data compression. Depending on the type of data one can reduce the disk space required by up to about 90%. A second space-saving technique was to combine calibration data, such as bias frames and flats. In many cases combined calibration data is just as good as t

It's actually a fascinating example that information theory and compression really are true. The calibration images such as bias images (a readout of the CCD with no effective exposure time) or dark images (a readout of the CCD with the shutter closed but with an exposure time like those of the actual sky observations) indeed contain little information and so compresses by factors of 4-5 with straightforward things like gzip. Regular images of the sky compress by approximately a factor of 2.

I know you're trying to be to be a smartass, but BMP<->PNG for example? Both are lossless but one takes much, much more space than the other. I'm sure there's other and probably better ways of compressing that data that's really specific to the application, like say FLAC is to music. Though I would guess that's already applied...

I'm no astronomer or physicist but isn't the fact that there is so much dark sky, even with powerful telescopes, suggestive of the presence of dark matter?

No, but the existence of dark sky *is* interesting.
Look up Olbers' paradox [wikipedia.org]. Any matter that respects the laws of thermodinamics (be it dust, interstellar gas or the newly defined "dark matter") should heat up to the temperature of its ambient medium and start radiating, so absorbtion can't be an explanation for the dark sky. There are however other th

No. Dark matter is not really related to the darkness of the night sky. Dark matter is so named because it doesn't interact with light one way or the other. It doesn't absorb light, it doesn't emit light, so it ends up being separate from the question.
But the question of why is the night sky dark is sometimes known as Olbers' Paradox [wikipedia.org] and has been used as one suggestion that the Universe is finite.

I used to think that until I started to work for astronomers. They actually take photos of noise, and then add noisy images together ("stacking") until pictures of interesting faraway things emerge from the noise. That's why a nightly sky survey is so useful - you can add together a few months of images and see stuff that you would never have seen in a single image.

30 TB per night sounds like a lot, but 1.5 TB drives are about AUD 350 each, retail. By 2016, I'd expect vendors to have released at least a 10 TB hard drive at that price point, and I wouldn't be surprised if we're using 30 to 50 TB drives.

So it all boils down to about $1000 per night of operation, or about $350K per year. Not exactly expensive for a science project. A single mars mission costs about $300M, but this telescope would generate more discoveries. That's not even considering that storage costs would continue to drop over the lifetime of the telescope, so the eventual total cost may be less than $100K per year. That's the salary of just one person!

The cost of the storage might be reasonable, but what about the performance aspect? 30TB per night sounds like a lot to store in one night...being generous and calling a night 12 hours - I'm probably wrong, but I make that 43200 seconds which is 694 MB/s. Without looking up any performance stats for hard drives, that sounds fairly easily attainable (too).

30 TB per night sounds like a lot, but 1.5 TB drives are about AUD 350 each, retail

Funny, but the idea of buying and installing twenty top-of-the-line new disks each day sounds like really big numbers to me...

Not to mention that they need backups. How many tapes do they have to buy? And data transfer, too. All those bytes are worthless if no one gets to see them, so they need at least 30 TB / day data link capacity.

What hard drives do they use? Gosh, how do their computers look? I mean if 1TB or 1.5TB hard drives really are the largest drives out there, then they would have to get like 150,000 hard drives!

Quantum do 112 drives in a 4U rack, which with controllers and raid, and assuming they've moved to 1.5TB drives since I last saw it (when it was 1TB) give you about a petabyte in a rack -- maid to reduce power consumption.

<nit>
That's 6.7 Meter effective diameter Telescope. The primary mirror has a diameter of 8.4m but the tertiary mirror (5.2m diameter) sits right in the middle of the primary, so its area needs to be subtracted from the primary. The area of the primary is pi*(8.4/2)^2 which is 55.4m^2 and the area of the tertiary is pi*(5.2/2)^2 which is 21.2m^2; a single mirror of that area would have a diameter of about 6.7m.</nit>

6.7 Meter Telescope To Capture 30 Terabytes Per Night

<grin>
Hey!! I thought information wanted to be free! And here they plan to go off and capture 30 TERAbytes? Each night? OMG!!!!11Eleventy!! Say it ain't so!!</grin>

Large Hadron Collider, Syntoptic Telescope Survey, Seismic Data Acquisition, Genome Decoding all use as much data capacity that exits. That now measures in the terabytes-per-day rate. Video tapes now have that capacity.